Now the woman is pulling the
crate at an angle. If she does the
same amount of work as before,
how much force must the woman
exert?
W = (F cos ș)ǻx
F = W/ (cos ș)ǻx
F = (360 J)/ (cos 39°)(3.0 m)
F = 150 N
6.2 - Interactive checkpoint: work
Sally does 401 J of work moving a
couch 1.30 meters. If she applies a
constant force at an angle of 22.0° to
the horizontal as shown, what is the
magnitude of this force?
Answer:F = N
6.3 - Energy
Before delving into some specific forms of energy, in this section we address the
general topic of energy. Although it is a very important concept in physics, and an
important topic in general, energy is notoriously hard to define.
Why? There are several reasons. Many forms of energy exist: electric, atomic,
chemical, kinetic, potential, and so on. Finding a definition that fits all these forms is
challenging. You may associate energy with motion, but not all forms of energy involve
motion. A very important class of energy, potential energy, is based on the position or
configuration of objects, not their motion.
Energy is a property of an object, or of a system of objects. However, unlike many other
properties covered so far in this textbook, is hard to observe and measure directly. You
can measure most forces, such as the force of a spring. You can see speed and decide
which of two objects is moving faster. You can use a stopwatch to measure time.
Quantifying energy is more elusive, because energy depends on multiple factors, such
as an object’s mass and the square of its speed, or the mass and positions of a system
of objects.
Despite these caveats, there are important principles that concern all forms of energy. First, there is a relationship between work and energy.
For instance, if you do work by kicking a stationary soccer ball, you increase a form of its energy called kinetic energy, the energy of motion.
Second, energy can transfer between objects. When a cue ball in the game of pool strikes another ball, the cue ball slows or stops, and the
other ball begins to roll. The cue ball’s loss of energy is the other ball’s gain.
Energy
Is changed by work